Inside a Sardinian Cork Factory

Published in the March/April 2018 issue of Tastes of Italia magazine.

Who doesn’t love the sound of a popping cork? It’s festive, classy, and it suggests a better wine. While many viable alternatives have emerged—twist-offs, synthetics, glass corks—natural cork is still the king of closures. A whopping 97% of consumers believe natural cork to be an indicator of quality, according to a 2017 poll by market-research group Wine Opinions. And 92% picked cork as their preferred bottle closure in a 2015 Wine Spectator poll. The runner-up—screwcaps—didn’t even come close: a paltry 4%. Imagine those stats in a political contest. Talk about a mandate.

And that’s despite the bad press during a rough patch for the cork industry. Cork taint surfaced as a serious issue in the 1980s, giving rise to the refrain “this wine is corked.” The culprit: a volatile compound called 2.4.6-trichloroanisole, or TCA, created by the interaction of mold, chlorine, and phenols. Minute amounts on the cork—just 6 nanograms per liter (ng/l)—can cause a bottle to smell like musty library books or wet cardboard. Lesser amounts can tamp down the wine’s aromas so it seems flat and lifeless.

It got so bad, many threw in the towel; cork usage fell from 95% worldwide in the 1990s to 70% today. Winemakers in Australia and New Zealand defected wholesale, fed up with getting the dregs (reportedly) from cork producers, with some saying half a delivery could be contaminated. Both countries are now screwcap loyalists: 90% of New Zealand wine and 70% of Australia’s is under aluminum twist-offs.

Cork producers got the message. Over the past two decades, they’ve cleaned up their act, putting quality controls in place that have significantly reduced the presence of TCA. Among bottles sold in the U.S., TCA levels have dropped by 81% since 2001, according to the California-based Cork Quality Council. With that danger now under control, prodigal winemakers are coming back to cork, resulting in a 4% growth in the industry since 2009.

To see how cork manufacturers have upped their game, I paid a visit to Italy’s largest, Sugherificio Peppino Molinas, producer of 1.5 million corks per year.

Cork trees on the road to Peppino Molinas, Italy's largest cork factory.

The miracle of quercus suber

Driving to the town of Calangianus on a winding descent in the Gallura region of Sardinia, I spot a grove of cork oaks. They’re unmistakable: the bark has been peeled off from branch to base, giving them the appearance of a shaved show poodle.

“Each cork stopper contains 800 million gas-filled cells. Scientists liken it to a closed-cell foam. I imagine it like tree bubble wrap.”

Quercus suber is a distinct type of oak. It doesn’t shed its leaves. It prefers coastal areas that ring the Mediterranean basin (Italy, France, Algeria, Tunisia) or flank the Atlantic (Portugal, Spain, Morocco). It can easily live for two centuries. And its thick bark has a unique cellular structure: a honeycomb of microscopic suberin cells, filled with air-like gas. Each cork stopper contains 800 million cells. Scientists liken it to a closed-cell foam. I imagine it like tree bubble wrap. It does in fact protect the trees from fire, and it’s one of nature’s best barriers against heat, sound, and liquid.

The backbreaking work of harvesting cork hasn’t changed over the ages. It takes a man and an axe. The trees must be 25 years old for the first “virgin” harvest, where a series of strategic cuts allows the worker to pull off huge rectangular planks without harming the tree. The bark grows back over the course of a decade, and the next harvest will be 9 to 12 years later. A tree can be harvested up to 17 times, making it the best renewable resource among bottle closures. Many oak savannas are protected ecosystems.

Sorting the bark for thickness, porosity, and appearance.

Season and boil

It’s June, halfway through the three-month window for cork harvest, and one flat-bed truck after another roars into the Molinas factory courtyard carrying piles of mossy planks harvested from the company’s 8,000 hectares (19,768 acres). Export manager Piera Cossu yells over the din: “Here they’re selecting the cork bark that was harvested last year,” pointing to men with thick gloves and tanned leathery faces who grab planks from a mound and toss them into piles, sorting for thickness, porosity, and appearance. The planks will be neatly stacked, then sit for a year in the open air to allow moisture levels to stabilize.

As we walk into the manufacturing plant, the noise level grows. There’s enough steam for another Blade Runner sequel, rising from tanks of boiling water where the bark soaks for an hour at 200ºF—the first step in its cleaning process. Plumped up by 20 percent, the planks rest for two days under a massive stone, flattening out. After their edges are trimmed, they’re ready to be turned into corks.

TCA sleuths

Avoiding TCA involves a combination of sophisticated machinery and trained human eyes and noses. Many of Molinas’s 250 employees have been working there for 25 to 30 years. “And they don’t move from their department,” Cossu adds, “so they know their raw material very well. You can’t just look at a cork bark and tell if it’s tainted; they cut it with a knife, and it’s a very hard job.”

When assessing a cross-section of cork, embedded slivers of wood are considered a flaw. That might seem strange—until you remember cork’s honeycomb structure. Hard oak is just the opposite: solid and inflexible. Other flaws on the watch list: “green cork,” with water inside; and yellow stain, a TCA hallmark.

We stop at a station manned by a human bloodhound: a lean, grey-haired worker who smells every plank before cutting it into strips. Cossu asks him for a reject. She sniffs. “It’s not very strong, but this has two defects: green cork and yellow stain,” she says, handing it to me. (My neophyte nose detects nothing.)

“What do you do with a piece like this?” I ask him. “Al mulino,” he says—to the mill.

In fact, only a small fraction of cork bark winds up as a “natural cork” stopper—that is, made from a single plug of cork. The leftover bits after punching and the lower-grade planks get ground into granules, which are composited into “technical corks,” in the industry jargon. Meanwhile, thin planks yield cork disks, which finish the ends of champagne corks and technical corks. Flawed planks become insulating wall panels, flooring, shoe heels, and other industrial products. “If you make a cork board with TCA taste, it’s not important,” Cossu wryly observes. Even cork dust gets recycled as biofuel, helping heat the Molinas plant. Nothing is wasted.

Punching the strips into single-piece natural corks.

Punch or composite

Cossu and I walk over to the punching department, where the natural corks take shape. Foot-long strips of cork are manually positioned in a machine that punches in a hollow metal cylinder, then extracts a single-piece cork. (In case you’re wondering, the punch goes parallel to the outer bark, not perpendicular, allowing room for those extra-long corks used in high-end wines.)

A mirror allows them to check the top side.

Two selections follow. Female employees visually check the corks as they roll by on a conveyor belt, including the top side and, in a facing mirror, the bottom, then sort them into different grades. The corks then pass through a kind of x-ray, which reveals any internal fissures that might cause problems down the road. Samples are taken to the lab, where gas chromatography hunts down TCA precursors.

Any remaining volatile compounds are eliminated using a Molinas-patented process called Genesis. This volatizes the molecules, then whisks them away using cycles of high-pressure steam and vacuum. Finally, the corks get washed in hydrogen peroxide or paracetic acid to clean and disinfect before being branded and packaged.

Technical cork stoppers follow a different process. Pellets of 3mm to 7mm are created from the remnants of the punching process as well as from planks containing wood. “There’s a process for cleaning the cork grains, so it rejects all the wood particles; you obtain a very clean cork granule,” says Cossu. After sorting, they’re steam-cleaned, then molded into a composite cork using an FDA-approved binding agent. Some models have a natural-cork disk affixed to the end that touches the wine. They’re oven-dried, polished, washed, and branded.

The new frontier: plasto-thermic microspheres inside micro-agglomerated cork-grain stoppers

The future of cork

Our last stop is the laboratory, where five staffers and one trainee work. There we find quality-control scientist Michele Addis checking some stoppers that are soaking in jars. After 24 hours, he explains, “a machine analyzes the liquid maceration to see if there are molecules of TCA or other volatile compounds. We also sniff all the samples that we put inside the water.” Addis, a 20-year veteran of Molinas, smells 600 corks a day. The nose knows.

He gestures towards other machines: “That’s to test the force needed to open a bottle. That one measures torsion—the strength needed to break the cork stopper.” There’s also the gas chromatographer, which analyzes compounds like TCA that can be vaporized without decomposition. In wide use since 2007, “the machine is made by companies like Hewlett Packard,” he says, “but we’ve improved the limits of the machine, so now we can detect 0.3 ng/l,” below the threshold of human perception. Their mass spectrometer can spot 0.2 ng/l. Both are critical tools in the fight against TCA.

Looking ahead, Addis is most jazzed about micro-agglomerated cork-grain stoppers, which Molinas markets under its FineCork and SmartCork lines. This is the industry’s growth sector. First, it’s easier to extract TCA from granulated cork, “so we can guarantee that 100% of the cork is under the perception level of TCA,” he states. Second, this product binds the granules using a technology that far surpasses alimentary glue. “It’s a nanotechnology using synthetic microspheres; they’re a shell with a gas inside,” Addis says. These plasto-thermic microspheres are something like cork’s air-filled honeycomb cells, but they expand like a balloon when heated. “When we put the cork in the oven, this kind of cell expands; that gives us the elasticity,” Addis says. “It’s the new frontier of cork—a combination of tradition and technology.”

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